1. Field of the Invention
Embodiments of the present invention generally relate to a reciprocating positive displacement pump utilized downhole within a wellbore to pump production fluid to a surface of the wellbore. More specifically, embodiments of the present invention relate to a drive mechanism for the downhole positive displacement pump.
2. Description of the Related Art
To obtain hydrocarbon fluids from an earth formation, a wellbore is drilled into the earth to intersect an area of interest within a formation. Upon reaching the area of interest within the formation, artificial lift means is often necessary to carry production fluid (e.g., hydrocarbon fluid) from the area of interest within the wellbore to the surface of the wellbore. Some artificially-lifted wells are equipped with sucker rod lifting systems.
Sucker rod lifting systems generally include a surface drive mechanism, a sucker rod string, and a downhole positive displacement pump. Fluid is brought to the surface of the wellbore by reciprocating pumping action of the drive mechanism attached to the rod string. Reciprocating pumping action moves a traveling valve on the positive displacement pump, loading it on the down-stroke of the rod string and lifting fluid to the surface on the up-stroke of the rod string. A standing valve is typically located at the bottom of a barrel of the pump which prevents fluid from flowing back into the well formation after the pump barrel is filled and during the down-stroke of the rod string.
The rod string of the sucker rod lifting system either includes several rods connected together or one continuous rod. Regardless of its make-up, the rod string provides the mechanical link of the drive mechanism at the surface to the positive displacement pump downhole. The typical rod string is constructed from steel or some other elastic material.
To access hydrocarbon fluid within a well, it is often necessary to drill a wellbore to a high depth within the formation, often termed a “deep well.” Pumping fluid from deep wells using a sucker rod lifting system is problematic for several reasons. First, the downhole positive displacement pump is submerged in the downhole fluid so that the positive displacement pump may fill with the surrounding production fluid upon reciprocation of the rod string, and because the fluid level of a deep well is typically located at a high depth within the wellbore, the rod string which connects the positive displacement pump to the drive mechanism must be long to access the fluids. A rod string of more than 10,000 feet is not uncommon. Therefore, the high length of the rod string as well as the material which makes up the rod string causes the rod string to weigh a large amount.
Additionally, the stroking motion of the rod string must be long to reduce the number of strokes required to displace the production fluid. The length of the motion of the rod string and the weight of the rod string cause the rod string to possess a high momentum at the end of the up-stroke and down-stroke, often causing the rod string to deform or break when motion is stopped between the up-stroke and down-stroke (at the “turnaround”). Specifically, the elastic nature of the material of which the rod string is constructed makes the rod string vulnerable to rod stretch, especially at the turnaround between the down-stroke and the up-stroke where the momentum of the rod string is most difficult to stop. Moreover, the stresses imposed on the rod string by a mismatch between the dynamic characteristics of the surface drive unit and the rod string may cause the rod string to break. This is particularly true when the rod string bounces up and down when attempting to switch the direction of the rod string at turnarounds between the up-stroke and down-stroke. Generally, rod string motion problems include premature rod string separation due to material fatigue, damage to the well tubing in which the rod string reciprocates and instantaneous rod string loads beyond the design limit due to suddenly applied loads from dynamic mismatch.
The downhole pump efficiency is affected by unfavorable rod string motion in other ways. A downhole pump needs time at the bottom of each stroke to fill with fluid and time at the top of each stroke to unload the fluid. Otherwise, the pump may cycle only partially filled. Rod string motion problems, including rod string damage, tubing damage, and only partial filling of the pump, increase as the load on and speed of the rod string are increased.
Sucker rod lifting systems include the additional problem when the well is pumped down to the point where fluid only partially fills the downhole pump barrel during the up-stroke of the rod string. On the next down stroke, the rod string, including the weight of the rod string and the fluid column, crashes into the partially-filled pump barrel and upon the standing valve. This crashing of the rod string is often termed “fluid pounding.” The condition at which fluid pounding occurs must be detectable by some kind of monitoring system to relay the condition to pump controls.
Another problem with deep-well sucker rod lifting systems is that the difference between the loading on the rod string during the up-stroke and the loading on the rod string during the down-stroke is severe. The load on the rod string during the up-stroke is much larger than the load on the rod string during the down-stroke because the drive mechanism must lift the hydrocarbon fluid from the wellbore on the up-stroke and must also contend with gravitational forces acting downward on the rod string while lifting the rod string for the up-stroke. In contrast, gravity aids the rod string motion during the down-stroke by acting in the same direction in which the rod string is moving, and fluid is not lifted, eliminating the additional weight of the fluid. This uneven loading requires a massive amount of horsepower for the drive mechanism to lift the rod string on the up-stroke, while limited horsepower is necessary for the rod string to fall into the wellbore on the down-stroke. Uneven loading in deep well pumps constitutes an inefficient use of horsepower because of the high amount of work expended in moving the rod string upward which is then not recovered upon the rod falling downward. Ideally, the rod load is evenly divided between the up-stroke and down-stroke of the pumping cycle to increase the efficiency of power use in the pumping unit.
FIG. 5A illustrates the rod string motion in graphical form for one drive mechanism currently used to cycle a rod string through and between the up-stroke and down-stroke, the crank and beam unit. Specifically, FIG. 5A shows a typical rod string motion graph for a crank and beam pump mechanical drive mechanism. The crank and beam pump mechanical drive mechanism articulates the rod string upward and downward within the downhole cylinder with a crank. The crank produces the sinusoidal rod string motion profile shown in FIG. 5A.
As shown in FIG. 5A, the turnaround point between the up-stroke and the down-stroke is at point T. The inter-cycle speed of the rod string during the up-stroke and down-stroke is sinusoidal and not constant, as indicated by the slope of the line representing the up-stroke and the down-stroke. Namely, the rod string moves at an uneven speed on the up-stroke and repeats the up-stroke motion on the down-stroke.
Dyno-card loading graphs illustrate loading on the rod string during a cycle, which includes the up-stroke, down-stroke, and turnarounds of the rod string between the up-stroke and down-stroke. The dyno-card graph represents load on the rod string versus position of a defined point on the rod string with respect to a defined point within or above the wellbore. Referring specifically to FIG. 6A, which is the dyno-card profile of the beam pump drive mechanism, the upper line between points J and K represents the loading on the rod string during the up-stroke, while the lower line between points J and K represents the loading on the rod string during the down-stroke. Points J and K represent the turnaround points of the rod string from the down-stroke to the up-stroke and from the up-stroke to the down-stroke, respectively.
The loading on the rod string is very erratic, as evidenced by the loading profile on the dyno-card graph. From point J to point K during the up-stroke, the rod string loading drastically increases to point P, then drastically decreases to point Q, only to increase and decrease again between points Q and K. The loading on the rod string at point P, which is the highest load on the rod string in this dyno-card profile, is higher than is healthy for the rod string. Similarly erratic, on the down-stroke, the loading drastically decreases to point R from point K, then increases to point S, then decreases again before increasing back to point J. This erratic loading on the rod string often stretches, breaks, or otherwise damages the rod string. Additionally, this erratic loading does not make efficient use of the horsepower which drives the drive mechanism.
Another drive mechanism explored for cycling the rod string through and between the up-stroke and the down-stroke is a gear-driven mechanical drive system having a mechanical counterbalance. As is shown in FIG. 5B, the mechanical drive system induces constant rod string motion except at the turnaround point T, so that inter-cycle speed is the same over the entire up-stroke as well as the entire down-stroke. Because the slopes of the lines on each side of the turnaround point T are not as severe as the slopes of the lines on either side of the turnaround point T of FIG. 5A, the inter-cycle speed of the rod string is lower for the system of FIG. 5B than for the system of FIG. 5A.
Despite the decrease in inter-cycle speed, the mechanical drive system with the mechanical counterbalance is generally an improvement over the crank and beam pump drive mechanism because of the more favorable loading profile evidenced in the dyno-card graph of FIG. 6B. The loading on the rod string does not erratically vary with position of the rod string; in fact, the loading on the rod string is nearly constant on the upstroke, which is generally from point L to point M and nearly constant on the down-stroke, which is generally from point N to point O. The turnaround point between the up-stroke and down-stroke is between points M and N, while the turnaround point between the down-stroke and the up-stroke is generally between points O and L.
While the inter-cyclic speed is good for this drive mechanism, as is evidenced by the favorable rod string motion profile shown in FIG. 5B, the loading on the rod string at the turnarounds of the rod string is not desirable. The undulations on the lines of FIG. 6B to the immediate right of the point L and to the immediate left of the point N represent the jarring which the rod string experiences at the abrupt stopping of motion and abrupt beginning of motion in the opposite direction of the rod string at the turnarounds. The jarring of the rod string also causes damage to the rod string, which may include breaking or stretching of the rod string. The amount of time the rod string spends at the top and the bottom of the stroke is not long enough to produce a good, smooth turnaround.
In gear-driven mechanical drive mechanisms, an electric motor rotates a gear reducer, and the gear reducer restricts the load and speed capacity of the mechanical drive mechanism. A problem with the mechanically-driven pumping units is that gear-driven pumping units are not very responsive to speed changes of the polished rod. Gear-driven pumping units possess inertia from previous motion so that it is difficult to stop the units or change the direction of rotation of the units quickly. Therefore, jarring (and resultant breaking/stretching) of the rod string results upon the turnaround unless the speed (strokes/minute) of the rod string during the up-stroke and down-stroke is greatly decreased at the end of the up-stroke and down-stroke, respectively. Gear-driven pumping units also are not sufficiently responsive to speed changes because of the tendency of the belts to burn up at abrupt speed changes and at high speeds and the torque limitations of gear reducers present in these systems. Decreasing of the speed of the rod string for such a great distance of the up-stroke and down-stroke decreases the speed of fluid pumping, thus increasing the cost of the well.
There is a need for a drive mechanism for a sucker rod positive displacement pump which efficiently uses horsepower provided to the drive mechanism. There is a further need for a drive mechanism which controls loading on the rod string to reduce rod string damage and to increase the amount of fluid volume pumped by the downhole pump. There is a yet further need for a drive mechanism which controls loading on the rod string during turnarounds between the up-stroke and the down-stroke, and vice versa. Finally, there is a need for a drive mechanism which is sufficiently responsive to alter the speed of motion of the rod string quickly.